In case where a short circuit fault occurs in one of an upper arm transistor and a lower arm transistor that form a bridge circuit, and the other transistor without the short circuit fault is turned on, a short-circuit current may flow into the transistors and a secondary fault may occur. Also when an output terminal for coupling a load makes a short-circuit to a power source line or when a short circuit fault occurs in windings of a motor as a load, a similar issue may occur.
JP-A-2009-71956 (corresponding to US 2009/0066402 A1) discloses a technique in which, in cases where a transistor is an insulated gate bipolar transistor (IGBT), when an on-command is given, a gate voltage slightly greater than a threshold value is applied to the IGBT to activate the IGBT in an active region, and the presence or absence of a short circuit fault is detected based on a detection current at the time. Then, after it is determined that a short circuit fault does not occur, a sufficiently high gate voltage is applied to activate the IGBT in a saturation region.
In the above-described configuration, when the gate voltage is too high to exceed a gate breakdown voltage VGES of the IGBT, the IGBT may be damaged or a life of the IGBT may be reduced. In a conventional gate driving circuit, a transitional increase of the gate voltage due to a surge is protected with a zener diode and the like, and a stationary increase of the gate voltage due to a change in a power source voltage is protected with a stabilizing power source. Both of a circuit that performs the short circuit protection and a circuit that performs the gate protection limits the gate voltage. However, because characteristics of the protections are completely different, an integrated circuit has not existed so far. The inventors of the present application found disadvantages of the conventional gate driving circuit, such as, an increase in a circuit size and a deterioration of a relative precision of protection levels of the short circuit protection and the gate protection.